Overview of Channel Multiplexing Techniques

This post gives a brief overview of channel multiplexing techniques like FDM, TDM etc and as to how they are used in computer communication.

Channel multiplexing is the process of splitting or sharing the capacity of a high speed channel/telecommunication link to form multiple low capacity/low speed sub-channels. Each such sub-channel can then be used by multiple end nodes as dedicated links. Multiplexing can usually be done in different domains like time, frequency and space (and even combinations of these).

Channel Multiplexing

For computer communication, though multiplexing techniques like TDM, FDM were initially used mainly in backbone links connecting multiple data exchanges, later they have percolated widely into the access/last mile links too, including inside home networks.

Time Division Multiplexing (TDM)

In TDM, a high speed data channel/link is made to carry data of multiple connections/end nodes in different time slots, in a round robin fashion. TDM is similar in concept to multitasking computers, where the main processor carries out multiple tasks simultaneously. In multitasking processors, though the processor executes only one task at any instant of time and keeps shuttling between multiple tasks in some order, because of the high speed in which it executes, each task thinks as though the processor is dedicated only to it.

Similary, in TDM, data of each connection is segmented into smaller units, so that they fit inside mini time slots. The link transmits these small units of data from multiple connections in a round robin fashion, periodically allotting a mini time slot for each user, in the time domain.

In TDM, the basic repeating unit is a frame. A TDM frame consists of a fixed number of time slots. Each time slot inside a frame carries data belonging to a specific end node/connection. Thus multiple logical sub-channels/links are created inside a single channel. It is also possible to give multiple slots within a frame to the same user, thereby having the provision of having different capacity sub-channels within the same link.

Assuming that there are “n” end users, each requiring a link with a capacity of X Kbps, then to successfully multiplex these each end users on a channel, the channel’s capacity needs to be atleast equal to n times X Kbps.

The Figure given below illustrates a sample TDM scheme with 4 users being served in a round robin fashion in the time domain.

An example TDM frame with 4 time slots serving 4 different users

In the example given in the figure, the TDM main channel servers a total of four users and hence creates four sub-channels. Each user’s data is carried in a specific slot inside each frame. For e.g. Channel-1’s (User 1) data is always carried in the first slot of each frame.

The basic principle of any TDM based protocol remains the same as described above, though there are multiple variants, based on

the transmission speed

number of frames generated per second

the number of time slots within each frame

the frame structure etc.

TDM is typically used in WAN digital transmission links, in both trunk and access networks. ISDN (Integrated Services Digital Network) is an example of a protocol using TDM at the access network, to connect home users to their neares ISP, using the local loop (telephone link). In ISDN, there are a total of 3 sub-channels, with two of them known as B-Channels (Bearer Channels), each with a capacity of 64Kbps being used to carry data and the third known as D-Channel with a capacity of 16Kbps being used to carry signalling information.

Standard T1/E1 serial links are classical examples of TDM based protocols and are used as trunk links between data exchanges. While T1 supports an aggregate rate of 1.54 Mbps with support for 24 sub-channels, each with a capacity of 64Kbps, E1 supports an aggregate rate of 2.08 Mbps with support for 32 sub-channels, each with a capacity of 64Kbps. TDM links with higher capacity include T2, T3 and SONET Optical links.

Time Division Duplex (TDD) is a form of TDM, where within the same TDM frame, some slots are used for uplink direction (end nodes to network) and some slots are used for downlink direction (network to end nodes), thereby enabling full duplex communication using the same TDM link.

Frequency Division Multiplexing (FDM)

In FDM, the spectrum (frequency range) of a high capacity link is divided into different non-overlapping intervals/carriers. Data of different end nodes are then modulated using these different carriers, so that the resultant signal of each end node occupies a different region in the frequency domain. Between each adjacent carriers, a small guard band is left unused, so as not to cause interference between closely separated carriers.

In FDM, at any instant of time, we would have electromagnetic signals corresponding to each node/sub-channel, unlike in TDM, where at any instant of time, the channel would only have electromagnetic signal belonging to one end node/sub-channel. This is shown in the diagram given below.

An example FDM with 6 different frequency carriers coexisting simultaneously in the time domain

Traditional FM Radio and Broadcast TV are classical examples of applications using FDM, where data belonging to each radio station/ TV channel is modulated over a different carrier, as shown in the diagram given below.

Three baseband channels modulated onto different frequency carriers to seperate them in the frequency domain

In computer communication, the concept of basic FDM and variants of FDM are widely used both in LAN and WAN environments. DSL and cable modem links are typical examples of physical layer protocols using FDM for achieving high data rates. In DSL, which also uses the standard telephone last mile local loop line, multiple sub-carriers, each with a bandwidth of 4KHz. are used to carry users data. The baseband region from 0 to 4KHZ is left for basic POTS voice calls. Above this, some number of sub-carriers are allotted for upstream traffic and a higher number of sub-carriers are alloted for downstream traffic. Similarly, cable modem has a separate frequency band for upstream traffic and a range of sub-carriers for downstream traffic.

An example diagram showing the sub-carrier spectrum allocation for POTS, DSL upstream and downstream directions are given in the diagram below:

FDM being used in ADSL, with different frequency sub-carriers for POTS, ADSL upstream and ADSL downstream (diagram courtesy Wikipedia)

In DSL, to achieve high data rates, a line coding technique like QAM is used on top of each sub-carrier. Thus both FDM and line coding techniques are combined at the physical layer to achieve high broadband data rates.

FDM is also used in some variants of Fast Ethernet (100 Mbps) and Gigabit Ethernet (1000 Mbps) LAN protocols, where multiple carriers are used to achieve the overall data rate supported by the underlying physical layer.

Variants of FDM

Wavelength Division Multiplexing (WDM) and DWDM (Dense-WDM) used in optical Networks, are based on principles similar to FDM, except that their carriers are based on different wavelengths instead of different frequencies

Frequency Division Duplexing (FDD) is a form of FDM, where some set of frequencies/carriers are used for carrying uplink direction traffic and some other set of frequencies are used for carrying downlink traffic, thereby enabling full duplex communication using FDM.

Spread Spectrum techniques are variants of FDM, where the data is carried or spread over a wide range of frequency spectrum. In normal FDM, a single carrier is used to carry data corresponding to an end node. But in Spread spectrum techniques, multiple carriers are used to carry data corresponding to an end node, with each carrier carrying a small piece of data. FHSS (Frequency Hopping Spread Spectrum), DSS (Direct Sequence Spread Spectrum) and OFDM (Orthogonal Frequency Division Multiplexing) are different types of spread spectrum techniques.

In FHSS, the frequency of the carrier varies from instant to instant, whereas in DSS, data is split into smaller units and simultanesouly carried by multiple carriers, as shown in the diagram given below

A FHSS where 8 different carriers are used, with data hopping on top of different carriers in a pre-determined order

CDMA (Code Division Multiple Access) is a form of DSS, where a codeword is combined with data to spread the signal over a wide range of spectrum.

OFDM is a form of DSS that is widely used in Wireless LAN protocols (802.11 a/g), wherein a set of carriers that are orthogonal (do not interfere with each other) are used to carry the data signal, as shown in the diagram below.

An OFDM scheme where 4 orthogonal frequency sub-carriers are used to spread the information over a wider spectrum

Orthogonal Frequency Division Multiplexing (OFDM) is used for both wired and wireless networks

Space Division Multiplexing (SDM)

In SDM, the same set of frequencies or same set of TDM signals are used in two different places that are geographically wide apart in space, so that one does not interfere with the other. Cellular communication, where the same set of carrier frequencies are reused (frequency reuse) in cells that are not close to one another is a classical example of SDM. Another example of SDM is the FM radio broadcast, where the same set of carrier frequencies are used in different cities that are geographically apart. These are examples of techniques where SDM and FDM are combined.

Combination of FDM,TDM and SDM

GSM (Global System for Mobile Communication) protocol combines both TDM and FDM, to achieve full duplex wireless communnication between the mobile handsets and the base stations. While one set of frequencies are used as base carriers from mobile handsets to base station, another set of non-overlapping frequencies are used as base carriers from base stations to mobile handsets, thereby making use of FDM principles. Within each carrier, GSM uses TDM, to carry voice and data belonging to multiple mobile users simultaneously, each in different time slots. Additionally the same set of carrier frequencies and TDM schemes are reused beyond a certain minimum distance, thereby making use of SDM. Thus GSM is an example protocol that uses FDM, TDM and SDM.